Reciprocal influence between the protein and lipid components of a lipid-protein membrane model

The crystallization of bacterial surface layer proteins (S-layer proteins) at phosphoethanolamine monolayers on aqueous (buffer) surfaces has been investigated with dual label fluorescence microscopy, FTIR spectroscopy, and electron microscopy. The phase state of the lipid exerts a marked influence on protein crystallization: when the surface monolayer is in the phase-separated state between the isotropic and anisotropic fluid phases, the S-layer protein is preferentially adsorbed at the isotropic phase. Protein crystals nucleate at the boundary lines between the coexisting lipid phases and crystallization proceeds underneath the anisotropic fluid. Crystal growth is much slower under the fluid lipid and the entire interface is overgrown only after prolonged protein incubation. In turn, as indicated by characteristic frequency shifts of the methylene stretch vibrations on the lipids, protein crystallization affects the order of the alkane chains and drives the fluid lipid into a state of higher order. Most probably, the protein does not interpenetrate the lipid surface monolayer and the coupling between protein and lipid occurs via the lipid head groups.